The present invention relates to a conveyor belt for underground mining that is reinforced by reinforcement layers and is provided with wear-resistant cover layers or cover plates made from an elastomeric material, whereby the elastomeric cover layers are comprised of a halogen-free rubber mixture, a main component of which is ethylene vinyl acetate rubber (EVM), and aluminum hydroxide as a fire-proofing agent.
Conveyor belts for underground mining made of elastomeric material must be fire-proofed due to the stringent fire prevention code for underground mining operations. Suitable fire-proofing agents are halogen compounds. However, they have the disadvantage that they release poisonous, caustic and corrosive fumes when burning
From a publication in "Gummi/Asbest/Kunststoffe", vol. 11/1978, pp. 863, it is known that fire-proofing of conveyor belts may be achieved by adding aluminum hydroxide instead of halogen containing additives to the rubber mixture. When burning, aluminum hydroxide forms only aluminum oxide and water vapor as decomposition products.
However, experiments have shown that aluminum hydroxide by itself is not adequate as a fire-proofing agent for conveyor belts in underground mining operations for fulfilling the extremely stringent requirements of the German coal mining organizations. Therefore, halogen containing additives must still be employed or halogen containing rubber mixtures must be used to pass fire-resistance tests and to receive a license for use of the respective conveyor belts in underground mining.
It has been demonstrated that ethylene vinyl acetate rubber (EVM) that contains aluminum hydroxide as a filler shows excellent fire-resistance. The higher the percentage of vinyl acetate in the rubber mixture the lower is the resulting calorific value of the rubber. Furthermore, EVM rubber has the additional advantage of burning without producing soot and emitting chlorine gas.
In order to obtain a sufficiently fire-resistant conveyor belt experiments concerning the use of aluminum hydroxide as a filler for EVM rubbers have been carried out. It was shown that the resulting EVM rubber is disadvantageously sticky which impairs processing on calenders during the manufacture of the cover layers and the rubberization of reinforcement layers.
In order to reduce the stickiness and to improve the processability it has been known from U.S. Pat. No. 4,243,576 to use a mixture of EVM rubber and EPDM (ethylene propylene diene monomer) rubber. EPDM rubber is characterized by a low smoke production during burning, however, its flammability characteristics are not as good as the those of EVM rubber.
It is therefore an object of the present invention to provide a conveyor belt for underground mining of the aforementioned kind which has an excellent fire-resistance whereby the rubber mixture employed has excellent processing characteristics.